1. Field of the Invention
The present invention relates to an electric corrosion preventing structure of a marine vessel propelling machine such as an outboard motor or the like.
2. Description of the Related Art
The marine vessel propelling machine of the outboard machine or the like is structured such that a propelling unit is tiltably coupled to a stern bracket fixed to a marine vessel via a swivel bracket, a tilting cylinder apparatus is interposed between the stern bracket and the swivel bracket, the tilting cylinder apparatus integrally forms a cylinder in a cylinder block, a rod slidably coupled to a rod guide fixed to the cylinder is inserted to an oil chamber in an inner portion of the cylinder, and a piston is fixed to an insertion end of the rod to the cylinder.
The tilting cylinder apparatus is provided with a tilt cylinder and a trim cylinder in the cylinder block, and actuates the propelling unit so as to tilt or trim. The tilt actuation by the tilt cylinder tilts the propelling unit within a tilt region against its own weight when a marine vessel is stopped, for raising the propelling unit above the water surface. The trim actuation by the trim cylinder tilts the propelling unit against a propelling force of a propeller within a trim region which has a smaller tilt angle than the tilt region, to adjust a cruising attitude of the marine vessel when the marine vessel is cruising.
In the marine vessel propelling machine mentioned above, since each of members of the marine vessel propelling machine is constructed of several different types of metal materials, an electrical potential difference is generated due to the metal material of each of the members coming into contact with sea water. The metal which has the higher ionization tendency is dissolved into the sea water, an effect known as electric corrosion.
Accordingly, in a conventional marine vessel propelling machine, as described in Japanese Unexamined Patent Publication No. 2005-329828 (patent document 1), in order to prevent the electric corrosion mentioned above, a metal such as a zinc or the like having a greater ionization tendency than a main material such as iron, aluminum alloy or the like of each of the members is provided as a sacrificial electrode (or a sacrificial anode, hereinafter, referred to as anode), and the anode is provided in a sea water contact portion of the propelling unit or the like, thereby preventing a corrosion of each of the other members.
However, in the conventional marine vessel propelling machine, the tilt cylinder and the trim cylinder which construct the tilting cylinder apparatus, particularly the trim rod of the trim cylinder is difficult to electrically connect to the anode provided in the propelling unit or the like, and electric corrosion is often generated. If the trim rod is not electrically connected to the anode, the anode can not be the sacrificial electrode for the trim rod.
For example, when the trim rod is constructed of an iron based material, a surface of the iron based material may appear uniform at first glance. However, the surface exhibits a different chemical component, structure, crystal orientation, oxide film, attachment or the like when observed by a microscope. A difference of electric potential is generated locally by this unevenness. In the case that the sea water comes into contact with the surface of the iron based material, an electric current flows from a higher electric potential to a lower electric potential in the iron based material, and flows from the lower electric potential to the higher electric potential in the sea water, and an ionization of the iron is generated in the portion having the lower electric potential. As a result, the portion having the lower electric potential corrodes in the surface of the trim rod.